The present invention relates to density compensators for oscillating-conduit Coriolis-type mass flowmeters.
In response to the need to measure the quantity of material being delivered through pipelines, numerous types of flowmeters have evolved from a variety of design principles. One of the more widely used types of flowmeters is based on volumetric flow. Volumetric flowmeters are at best inaccurate in determining the quantity of material delivered, where the density of the material varies with temperature of feedstock, where the fluid being pumped through the pipeline is polyphase such as a slurry or where the fluid is non-Newtonian such as mayonnaise and other food products. In addition, the metered delivery of liquid components for chemical reactions, which are in effect mass reactions where proportions are critical, may be poorly served by volumetric flowmeters.
A mass flowmeter, on the other hand, is an instrument that provides a direct indication of the quantity of mass, as opposed to volume, of material being transferred through the pipeline. Measurement of mass in a moving stream requires applying a force to the stream and detecting and measuring some consequence of the resulting acceleration.
One class of mass measuring flowmeters is based on the well-known Coriolis effect. Examples of Coriolis-type mass flowmeters are described in U.S. Pat. No. 4,891,991 to Mattar et al., entitled "Coriolis-Type Mass Flowmeter," issued on Jan. 9, 1990, and U.S. patent application Ser. No. 07/446,310 filed Dec. 5, 1989 by Hussain et al., both assigned to the assignee of the present invention and incorporated herein by reference in their entirety.
Many Coriolis-type mass flowmeters induce a Coriolis force by oscillating the pipe sinusoidally about a pivot axis orthogonal to the length of the pipe. In such a mass flowmeter, Coriolis forces are exhibited in the radial movement of mass in a rotating conduit. Material flowing through the pipe becomes a radially travelling mass which, therefore, experiences an acceleration. The Coriolis reaction force experienced by the travelling fluid mass is transferred to the pipe itself and is manifested as a deflection or offset of the pipe in the direction of the Coriolis force vector in the plane of rotation.
A major difficulty in these oscillatory systems is that the Coriolis force, and therefore the resulting deflection, is relatively small compared not only to the drive force but even to extraneous vibrations. On the other hand, an oscillatory system can employ the inherent bending resiliency of the pipe itself as a hinge or pivot point for oscillation to obviate the need for separate rotary or flexible joints, which improves mechanical reliability and durability. Moreover, an oscillatory system offers the possibility of using the resonant frequency of vibration of the conduit itself to reduce the drive energy needed.
Energy is supplied to the conduits by a driving mechanism that oscillates the conduits by applying a periodic force. A typical type of driving mechanism is exemplified by an electromechanical driver, which exhibits motion proportional to an applied voltage. In an oscillating flowmeter the applied voltage is periodic, and is generally sinusoidal. The period of the input voltage (and, hence, the driving force) and the motion of the conduit is chosen to match one of resonant modes of vibration of the conduit. As mentioned above, this reduces the energy needed to sustain oscillation.
The Coriolis force resulting from oscillation of the conduit and mass flow is measured indirectly by sensors disposed on the flowmeter conduit. Like the driving force, the Coriolis force is periodic, having the same frequency as the driving force. The motion arising from the Coriolis force is, however, mathematically orthogonal to the drive motion. This relationship prohibits the drive motion from coupling with the Coriolis motion in an ideal flowmeter. In certain flowmeter designs, however, the Coriolis motion can couple with a second mode of vibration of the conduit, different from the drive mode. The amount of coupling between the Coriolis motion and the second mode of vibration is generally referred to as the amplification factor, which has a magnitude that depends on the ratio of Coriolis mode frequency and the resonant frequency of the second mode of vibration. The amplification factor should be a constant for a given system and must be known for the liquid flowing in the flowmeter.